It would certainly be more common in Classical usage to say facultas veritatis cognoscendae, although the way it’s written isn’t necessarily un-Classical; you do see the gerund taking an object now and then. For the last part, voluntas has to be nominative because it’s the subject of minuitur (or am I missing something?)
Here’s a bizarre sentence I found in the Catechism:
Ecclesia saeculorum decursu adducta est ad plenam animae humanae cum eius intellectus et voluntatis operationibus atque corporis humani Christi realitatem confitendam.
the Church was led over the course of centuries to confess the full reality of Christ’s human soul, with its operations of intellect and will, and of his human body.
I had to check the English translation. It’s the first time I’ve ever seen ad and a gerundive separated so far apart, not to mention the giant separation between plenam and realitatem. Why would anyone do that? Perhaps the translator delayed realitas until the end so that it could go with both animae humanae (the full reality of Jesus’ human soul) and corporis humani (the reality of Christ’s human body)? But it still strikes me as awkward.
Latin is great at this sort of thing and better than English. Look how detached in English is “reality” and “of his human body” and how, in the second sentence “full” and “reality” can only belong together so the intervening complex clause must attach to both together. Open the box with “plenam”; put the lid on with “realitatem confitendam”.
the Church was led over the course of centuries > to confess the full reality > > of Christ’s human soul, with its operations of intellect and will, and of his human body> .
the Church was led over the course of centuries > to the full > > of Christ’s human soul, with its operations of intellect and will, and of his human body > > reality confess> .
Mirum est Latinum, melius quàm Anglicum, hoc pro rerum genere. Nota quàm longè anglicè inter se separata sunt “reality” et “of his human body”. Secundo In exemplo clarum est clausulam contortam inter “full” et “reality confess” utris conjunctis servare. Cista per “plenam” aperiatur; operculum per “realitatem confitendam” reponatur.